foreign-slot-value (ptr type slot-name)

ptr -- A pointer to a structure.
type -- A foreign structure type.
slot-name}{A symbol naming a slot in the structure type.} @argument[object -- The object contained in the slot specified by slot-name.

For simple slots, foreign-slot-value returns the value of the object, such as a Lisp integer or pointer. In C, this would be expressed as ptr->slot.

For aggregate slots, a pointer inside the structure to the beginning of the slot's data is returned. In C, this would be expressed as &ptr->slot. This pointer and the memory it points to have the same extent as ptr.

There are compiler macros for foreign-slot-value and its setf expansion that open code the memory access when type and slot-names are constant at compile-time.

  (defcstruct point
    "Pointer structure."
    (x :int)
    (y :int))


  CFFI> (with-foreign-object (ptr 'point)
          ;; Initialize the slots
          (setf (foreign-slot-value ptr 'point 'x) 42
                (foreign-slot-value ptr 'point 'y) 42)
          ;; Return a list with the coordinates
          (with-foreign-slots ((x y) ptr point)
            (list x y)))
  => (42 42)